The decision by Express Scripts to make the AbbVie hepatitis C treatment the exclusive option for patients with the most common type of the virus caused a ruckus on Wall Street, although the reaction among some doctors and patient advocates was mixed. For Gilead, the market leader in hepatitis C drugs, the move by the big pharmacy benefits manager was the proverbial bitter pill. Gilead shares plunged 14% to $92.90, wiping out more than $20 billion in market capitalization over expectations of a big hit to sales. “The impact of the deal is clearly a negative for Gilead,” writes Sanford Bernstein analyst Geoff Porges in an investor note. But he maintains that impact may be blunted because the deal may only affect 8% of Americans infected with the disease, and even then not all will be eligible for the new AbbVie drug called Viekira Pak.